The measurement or sensing of the mass of air-borne particles and particulate materials by means of a quartz (piezoelectric) crystal micro balance is well known in the art. The piezoelectric crystal is forced to vibrate, while the particles being sensed are caused to impinge upon the vibrating crystal surface. The particles are captured by the crystal, and the vibrating frequency of the crystal is caused to decrease as a result of their capture. The mass of the collected particles which is proportional to this frequency change, is then easily measured.
One of the problems with obtaining accurate measurements of the particle mass, however, is the "positive" capture or collection by the crystal of the air-borne particles. In order to provide a proper particle-captive surface for the vibrating crystal, the crystal has in the past been coated with tacky or adhesive substances, such as silicon oil and grease-like substances. The object of these tacky coatings is to rigidly couple or adhere the impinging particles to the crystal surface, so that they can sufficiently follow the movement of the crystal surface. When this condition is achieved, the "slaved" particles will alter the oscillation of the vibrating crystal. Such a teaching can be found in the art with reference to the U.S. patents to: R. L. Chuan, entitled: Apparatus for Sensing Air-borne Particulate Matter; U.S. Pat. No. 3,715,911; issued: May 11, 1970; and W. H. King, Jr., entitled: Piezocrystal Fluid Analyzer; U.S. Pat. No. 3,266,291; issued: Aug. 16, 1966.
Tacky layers will usually provide the necessary adhesiveness to capture impinging particles, but have been found to give erroneous results. This is probably due to the fact that tacky materials are often too soft to rigidly couple the captured particles to the surface of the crystal, such that they follow the crystal movement and influence or alter the oscillations. If made more rigid so as to slave the particles to the crystal movement, these layers will not properly collect or capture the particles, such that the use of tacky layers can never be designed to be entirely successful.
Other problems with these tacky layers exist which further discourage their use. Such layers are designed to be thin so as to reduce layer shearing and crystal damping effects. When these layers are made thin, however, they are too thin to "positively" capture the incident particles; i.e., to hydrodynamically stagnate the impinging particles.
The invention is concerned with providing a surface for a mass-sensing piezoelectric crystal which will "positively" capture incident particles, such that: (1) they will not bounce when they impinge; and (2) their mass will be rigidly coupled to the oscillating crystal surface.